Why is the scale of the model I try to 3D print so small?
STL doesn't use metric units, if your model is 1BUx1BUx1BU (BU=Blender Unit) it will be 1mm x 1mm x 1mm in size, despite the fact that 1BU is 1m in Blender.

You can fix it in two ways, either by manually scaling your object in the scene and assuming that 1 unit is 1 mm (after exporting to STL) or you can set the scale in the Blender's STL exporter to 1000.

I prefer the first method, as you don't have to fiddle with the scaling settings and you just model in mm.

Also, always make sure you 'apply the scale' of your object before exporting via Ctrl+A > Scale.